Noise limiters for electronic high-frequency oscillators

ABSTRACT

PULSE-OPERATED ELECTRONIC HIGH-FREQUENCY OSCILLATORS, SUCH AS MAGNETRONS, EMPLOYED IN A RADIO TRANSMITTING SYSTEM WHICH APPLIES TO THE OSCILLATOR A CONSTANT DC PEDESTAL VOLTAGE OF A FIRST MAGNITUDE AND WHICH SUPERPOSES ON SAID PEDESTAL VOLTAGE AN OPERATING VOLTAGE PULSE OF A SECOND MAGNITUDE FOR GENERATING RF TRANSMISSION ENERGY, ARE OPERTATED MORE STABLY WITH REDUCED NOISE BY A QUARTER-WAVE TRANSMISSION-LINE STUB BRANCHINGLY COUPLED TO THE OUTPUT TRANSMISSION LINE AN OPTIMIZED DISTANCE FROM THE OSCILLATOR. THE STUB IS OPEN-CIRCUITED AT ITS FREE END AND A NORMALLY-OPEN SWITCH, WHICH IS CLOSED IN THE PRESENCE OF RF POWER ABOVE A CERTAIN THRESHOLD, IS COUPLED ACROSS THE OPEN END.

United States Patent O 3,718,867 NOISE LIMITERS FOR ELECTRONIC HIGH- FREQUENCY OSCILLATORS Robert Tenenholtz, Framingham, Mass., assignor to Microwave Associates, Inc., Burlington, Mass. Filed Feb. 17, 1972, Ser. No. 227,176 Int. Cl. H03b 9/10 U.S. Cl. 331--91 9 Claims ABSTRACT F THE DISCLOSURE -Pulse-operated electronic high-frequency oscillators, such as magnetrons, employed in a radio transmitting system which applies to the oscillator a constant DC pedestal voltage of a rst magnitude and which superposes on said pedestal voltage an operating voltage pulse of a second magnitude for generating RF transmission energy, are operated more stably with reduced noise by a quarter-wave transmission-line stub branchngly coupled to the output transmission line an optimized distance from the oscillator. The stub is open-circuited at its free end and a normally-open switch, which is closed in the presence of RF power above a certain threshold, is coupled across the open end.

BACKGROUND OF THE INVENTION A pulse-operated magnetron is operated in `a mode which requires it to emit pulses of radio-frequency energy spaced by time intervals during which the magnetron is essentially turned ofl, or not oscillating. Each pulse is an event requiring that the magnetron be turned on and then off. Each time the magnetron is turned on, it is desired that it go from off to on as soon as possible, and that it reach the full-on condition in a stably repeatable fashion and time intermal, the latter being desirably a stably-repeatable minor fraction of the pulse length. The state-of-the-art is such that magnetrons do not start in an ideal stably-repeatable fashion, and the variation from pulse-to-pulse is known as starting-time jitter. Starting time jitter causes undesired noise; that is, emission of radio frequency energy at frequencies other than the desired operating frequency of the magnetron. These problems are exemplified by magnetron oscillators; they are encountered also in other forms of electronic highfrequency oscillators when the latter are operated in a mode which requires that they be repeatedly started and stopped.

GENERAL NATURE OF THE INVENTION Standard magnetrons normally require a modulating or trigger voltage pulse of, for example, 700i volts, to set them into oscillation. The generator of such a pulse must therefore be built to withstand voltages of that level. The present invention is directed to a system having a power source which provides a constant D.C. pedestal voltage of a lfirst magnitude, such as 500 volts, which is not yadequate to set the magnetron into oscillation, and which super-poses on said pedestal voltage a modulating or trigger pulse voltage of -a second magnitude, for example, 200 volts, to set the magnetron into oscillation. This system has the advantage that it can use a smaller and cheaper modulator. But the constant presence of the D.C. pedestal voltage produces excessive noise throughout the interpulse intervals. In a radar system, for example, this noise degrades receiver performance. EFor purposes of this invention, noise is defined as emission of RF energy at all frequencies other than the desired operating frequency of the magnetron.

There is, therefore, a need to reduce the noise as well as starting time jitter of pulse-operated, electronic, high- ICC frequency oscillators. The principal object of this invention is to provide a means to achieve these improvements. Another object is to provide such means which is passive in nature, in that no external or additional source of energy is required for its operation. Additionally, it is an object to provide such means which is simple and reliable in design and construction. A specific object is to provide about 20 db isolation between a magnetron and its output circuit during interpulse intervals so as to substantially eliminate interpulse noise.

An exemplary embodiment of the invention is described with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates one embodiment of the invention;

FIG. 2 graphically illustrates a scheme of voltages applicable to the magnetron of FIG. l;

FIG. 3 is a front view of a microwave component intended for insertion between a magnetron and its load according to the invention;

FIG. 4 is a partial section on line 4 4 of FIG. 3; and

FIG. 5 illustrates a detail of FIG. 4.

In FIG. 1, an oscillator 10 is coupled to one end 11 of a iirst section of transmission line 12. A second section of transmission line 13 is branchngly coupled at one end to the lirst line 12 in a region 1S which is located a distance d from the end 11 which is coupled to the oscillator. The distance d is optimized for stability of the oscillator at the desired frequency of operation. Typically d is approximately equal to:

in m 4 2 where:

gi1=the wavelength in the first transmission line of electric wave energy at the center frequency of the Iband in which the oscillator operates; and

n=an integer.

When the oscillator is a magnetron, the susceptance of the output window is taken into account in adjusting this dimension d.

The second section 13 is approximately one-quarter guide wavelength long (again, for the same center frequency) or an odd multiple thereof. In the example illustrated, the rst section 12 is a waveguide, and the second section 13 is a coaxial line having a center conductor 16 connected at the remote end 17 to the outer conductor through a semiconductor diode 18. The diode 18 is a semiconductor limiter, (e.g.: Microwave Associates Inc. Type No. 47624) which is normally open circuited, but has a threshold such that when RF energy at the oscillator frequency is impressed across it at a power level above a given reference it conducts and effectively connects the center and outer conductors together at the remote end 17. The remote end 19 of the first section 12, beyond the region 15 relative to the end 11 coupled to the oscillator, is a terminal end for coupling to a load (not shown).

When the diode 18 is open-circuited (not conducting),

-as it will be during the initial starting time of the oscillator 10, the output load presented to the oscillator is unmatched, since the open-circuit at the remote end 17 of the second section 13 appears electrically as a shortcircuit in parallel with the waveguide first section 12. Under this condition, RF power from the oscillator is reected back into the oscillator. When the oscillator power builds up to a sufciently high level to switch the diode 18 to a conducting state, the remote end 17 becomes a short circuit, which appears electrically as an open circuit across the waveguide iirst section 12, and the oscillator then has a matched output.

For positive switching of the limiter diode 18v a rectifier diode 21 is provided, connected in circuit with an RF tap 22 in the waveguide section between the input end 11 and the switch region 15, the rectified output of which is supplied, via a conductor 28 extending through the waveguide wall by way of an RF by-pass capacitor 42, to the coaxial line center conductor 16, which extends through the waveguide wall via an RF by-pass capacitor 23. A source 24 of constant DC voltage is applied between the anode 25 and cathode 26 of the magnetron; this source is represented as a battery but it will be recognized that other forms of DC voltage may be used. A source of voltage pulses P' represented by a block P is connected in series between the battery 24 and the cathode 26. The voltage from the battery alone is not suicient to produce RF energy from the magnetron which will switch the limiting diode 18 to the conductive state, but when the voltage pulse P is added to the DC voltage the rectifier diode 21 produces a large enough voltage to bias the limiter diode 18 to a conducting condition.

This is illustrated in FIG. 2 where reference line 30 is the zero-voltage line, reference line 31 labelled 500V is the D.C. voltage level produced by the battery 24, and pulses P' are shown to have a voltage magnitude of about 180 volts, to make a total of 680 volts applied between anode and cathode of the magnetron to set it into oscillation. At that voltage tap 22. picks up sufficient RF energy for rectifier diode 21 to produce adequate DC bias to switch the limiter diode 18 to a conducting state, and an electrical short circuit appears at the end 17 of branch coaxial line 13. Reference line 33 illustrates noise at about 40 dbm. in the interpulse periods, and RF pulses 34 of about 40 watts power in the output 19 when the modulator pulses P are applied.

The limiter diode 18 may be a PIN type, preferably having a thin base, or a varactor. One suitable commercial type is mentioned above. The rectifier diode 21 may be a Schottky diode, or a point contact diode. The inductor 36 may have a value of 100 ph., and the resistor 37 may be 33 ohms. Alternatively, the resistor 37 may be used alone, at a value about 200 ohms, or the inductor 36 may be used alone at a value about 200 nh., depending on engineering considerations, such as insertion loss, output power, pulse width and switching time.

When the magnetron pulse ceases the choke 36 reverses lvoltage and back-biases the diode 18. This improves isolation. When the voltage across the choke reduces to a very low level (e.g.: 0.7 volts) diode 21 if it is a Schottky type stops conducting. This leaves the charged inductor 36 and by-pass capacitor 23 which will oscillate at low frequency (relative to microwave frequencies). The resistor 14, value about 1K loads this circuit to prevent oscillation.

The invention is realized in a single microwave component that is illustrated in FIGS. 3, 4 and 5. FIG. 4 shows a sectional side view of the component, in which parts that are found in FIG. l have the same reference characters. The component is made in a single body 50i, which may be of brass or aluminum, one face 51 of which is intended to mate with the output transmission line of a magnetron (or the output of another suitable oscillator), and the other face 52 of which is intended to function as the remote end 19 of the transmission line 11-15-19 shown in FIG. 1. This waveguide, labelled 55 in FIG. 4, extends between the faces 51 and 52.

The bias rectifier diode 21 is housed in a tubular housing 4f) made of a brass tube 41 inserted in a bore 43 in the body 50. The tap 22 is connected at one end to this tube 41. The connecting wire 28 from the rectifier diode 21 to the coaxial line center conductor 16 passes through the RF by-pass capacitor 42 fitted at one end of the tube 41.

The center conductor 16 is held between two RF choke subassemblies 61 and 62 of cylindrical form, the -first of which, 61, is illustrated in axial section in FIG. 5. The choke structure, well known in form, is made for example of brass, and a plug 63 of anodized aluminum holds the center conductor at the end 16' connected to the wire 2S. The plug 63 is DC insulated owing to the anodizing coat on it. The lower choke assembly 62 is not described in detail, since choke structures are Within the competence of those skilled in the art; instead the location of the remote wall 17 of the coaxial line 13 is shown.

The face view of FIG. 3 shows mounting holes 65, which may be internally threaded, for coupling to transmission line waveguide going beyond the remote end 19 shown in FIG. l. The lower choke sub-assembly 62 may be held in place with a bolt 66. Clearly, the choke subassemblies, being cylindrical in form, are supported in bores, not referenced, in the component body 50. The choke 36 and resistor v37 are externally mounted, and can therefore be changed to use one of the alternatives mentioned above.

The invention is applicable to oscillators of many forms, for example, gallium arsenide bulk-effect oscillators working in the L.S.A. mode. Also, the invention is applicable to all types of transmission lines and transmission-line syste-ms, including for example waveguides, coaxial lines, strip transmission lines, and the like, and combinations thereof. Moreover, limiters of other forms than those illustrated can be used, such as ferrite, or yttrium iron garnet, as examples. It is to be noted that gas-type ATRs ywould be particularly suitable for high-power applications where diode and other solid-state limiters now suffer from power limitations. Thus, this specification is intended to illustrate, but not to limit, the scope of the appended claims.

I claim:

1. In a pulse-modulated radio transmitter system employing a pulse-operated high-frequency oscillator such as a magnetron as a source of RF transmission energy and having a transmission path for propagating said energy away from said magnetron, the combination of a power source which applies to said magnetron a constant DC pedestal voltage of a first magnitude and which superposes on said pedestal voltage an operating voltage pulse of a second magnitude for generating said RF transmission energy, and means for blocking propagation in said path of RF energy produced in said magnetron due to the constant presence of said pedestal voltage during absence of said operating voltage pulse, said blocking means comprising switch means having two mutuallyexclusive states coupled to said path for establishing therein a substantial impedance mismatch to said RF energy when said switch means is in a rst of said states and a substantially impedance-matched condition when said switch means is in the second of said states, said switch means being normally in said first state in the presence of said pedestal voltage without said operating voltage pulse and settable into said second state in the simultaneous presence of both said voltages.

2. A system according to claim 1 in which said first state is open andrsaid second state is closed.

3. A system according to claim 1 including a second transmission path branchingly connected to said firstnamed path, said switch means being coupled in said second path so as to receive imposed across it a potential responsive to RF voltage from said magnetron, said switch means being effectively open-circuit in the presence of said potential responsive to RF voltage due to said pedestal voltage alone, said switch means being effectively closed-circuit in the presence of said potential responsive to RF voltage due to both said pedestal and pulse voltages together.

4. A system according to claim 3 in which said switch means issemiconductor diode limiter means.

5. A system according to claim 1 in which said switch is voltage-responsive, and including means responsive to said operating Voltage pulse to set said switch means into said second state.

6. A system according to claim 3 in which said switch means is voltage-responsive and including bias-voltage supply means coupled to said switch means to set said switch means in said closed-circuit state.

7. A system according to claim 6 in which said bias voltage supply means includes a tap for said RF voltage and rectifier means to provide a bias voltage proportional to said RF voltage.

8. A system according to claim 5 in which said setting means includes reactive circuit components, comprising resistance means to inhibit oscillations tending to arise in said reactive components upon termination of said operating voltage pulse owing to energy stored in one or more of said reactive components.

9. A system according to claim 5 including a tap for said RF energy and rectifier means coupled to said tap for generating a voltage to set said switch means into said second state, circuit means con ected to said tap including reactive components, and resis nce means connected to said circuit means for inhibiti oscillations tending to arise in said reactive components on termination of said operating voltage pulse owing to energy stored in one or more of said reactive components.

References Cited UNITED STATES PATENTS 3,628,183 12/1971 Strenglein 331-96 JOI-IN KOMINSKI, Primary Examiner U.S. C1. X.R. 331-87 

